Unnatural cyclic α-amino acids play an important role in the search for biologically active compounds and macromolecules. Enantiomers of natural amino acids with a D configuration are not naturally encoded, but can be chemically synthesized. The crystal structures of two enantiomers obtained by a method of stereoselective synthesis, namely (5R,8S)-8-tert-butyl-7-methoxy-8-methyl-9-oxa-6-azaspiro[4.5]decane-2,10-dione, (1), and (5S,8R)-8-tert-butyl-7-methoxy-8-methyl-9-oxa-6-azaspiro[4.5]decane-2,10-dione, (2), both C14H21NO4, were determined by X-ray diffraction. Both enantiomers crystallize isostructurally in the space group P21, with one molecule in the asymmetric unit and with the same packing motif. The crystal structures are stabilized by C—H...O hydrogen bonds, resulting in the formation of chains along the [100] and [010] directions. The conformation of the 3,6-dihydro-2H-1,4-oxazin-2-one fragment was compared with other crystal structures possessing this heterocyclic moiety. The comparison showed that the title compounds are not exceptional among structures containing the 3,6-dihydro-2H-1,4-oxazin-2-one fragment. The planar moiety was more frequently observed in derivatives in which this fragment was not condensed with other rings.
An Efficient Synthesis of Tetrodotoxin
